Gan-based schottky barrier diode with algan surface layer

ABSTRACT

A Schottky diode and method of fabricating the Schottky diode using gallium nitride (GaN) materials is disclosed. The method includes providing an n-type GaN substrate having first and second opposing surfaces. The method also includes forming an ohmic metal contact electrically coupled to the first surface, forming an n-type GaN epitaxial layer coupled to the second surface, and forming an n-type aluminum gallium nitride (AlGaN) surface layer coupled to the n-type GaN epitaxial layer. The AlGaN surface layer has a thickness which is less than a critical thickness, and the critical thickness is determined based on an aluminum mole fraction of the AlGaN surface layer. The method also includes forming a Schottky contact electrically coupled to the n-type AlGaN surface layer, where, during operation, an interface between the n-type GaN epitaxial layer and the n-type AlGaN surface layer is substantially free from a two-dimensional electron gas.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/300,009, filed on Nov. 18, 2011, the disclosure of which isincorporated by reference herein in its entirety for all purposes.

The following regular U.S. patent application is incorporated herein inits entirety for all purposes:

-   -   Application Ser. No. 13/300,028, filed Nov. 18, 2011, entitled        “GAN-BASED SCHOTTKY BARRIER DIODE WITH FIELD PLATE” (Attorney        Docket No. 93444-819312(001800U5)).

BACKGROUND OF THE INVENTION

Power electronics are widely used in a variety of applications. Powerelectronic devices are commonly used in circuits to modify the form ofelectrical energy, for example, from AC to DC, from one voltage level toanother, or in some other way. Such devices can operate over a widerange of power levels, from milliwatts in mobile devices to hundreds ofmegawatts in a high voltage power transmission system. Despite theprogress made in power electronics, there is a need in the art forimproved electronics systems and methods of operating the same.

SUMMARY OF THE INVENTION

The present invention relates generally to electronic devices. Morespecifically, the present invention relates to techniques for providinga Schottky barrier diode using III-nitride semiconductor materials andhaving a surface layer that modifies the performance of the Schottkybarrier diode. Merely by way of example, the invention has been appliedto methods and systems for manufacturing Schottky barrier diodes usingone or more gallium-nitride (GaN) based epitaxial layers and an aluminumgallium nitride (l1GaN) surface layer. The methods and techniques can beapplied to create diodes for a variety of applications that can benefitfrom the low leakage current of the diodes.

According to an embodiment of the present invention, a method offabricating a Schottky diode using gallium nitride (GaN) materials isprovided. The method includes providing an n-type GaN substrate having afirst surface and a second surface, the second surface opposing thefirst surface. The method also includes forming an ohmic metal contactelectrically coupled to the first surface of the n-type GaN substrate,forming an n-type GaN epitaxial layer coupled to the second surface ofthe n-type GaN substrate, and forming an n-type aluminum gallium nitride(AlGaN) surface layer coupled to the n-type GaN epitaxial layer. TheAlGaN surface layer has a thickness which is less than a criticalthickness, where the critical thickness is determined based on analuminum mole fraction of the AlGaN surface layer. The method alsoincludes forming a Schottky contact electrically coupled to the n-typeAlGaN surface layer, where, during operation, an interface between then-type GaN epitaxial layer and the n-type AlGaN surface layer issubstantially free from a two-dimensional electron gas.

According to another embodiment of the present invention, asemiconductor device is provided. The semiconductor device includes ann-type GaN substrate having a first surface and a second surface, thesecond surface opposing the first surface. The device also includes anohmic metal contact electrically coupled to the first surface of then-type GaN substrate, an n-type GaN epitaxial layer coupled to thesecond surface of the n-type GaN substrate, and an n-type aluminumgallium nitride (AlGaN) surface layer coupled to the n-type GaNepitaxial layer. The AlGaN surface layer has a thickness which is lessthan a critical thickness, where the critical thickness is determinedbased on an aluminum mole fraction of the AlGaN surface layer. Thedevice also includes forming a Schottky contact electrically coupled tothe n-type AlGaN surface layer, where, during operation, an interfacebetween the n-type GaN epitaxial layer and the n-type AlGaN surfacelayer is substantially free from a two-dimensional electron gas.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentinvention provide diodes having lower leakage current and increasedreverse breakdown voltage in comparison with conventional devices.Additionally, embodiments can utilize materials in surface layers thatare more stable and easier to handle in manufacturing than conventionalmaterials.

Another advantage provided by embodiments of the present invention overconventional devices is based on the superior material properties ofGaN-based materials. Embodiments of the present invention providehomoepitaxial GaN layers on bulk GaN substrates that are imbued withsuperior properties to other materials used for power electronicdevices. High electron mobility, μ, is associated with a givenbackground doping level, N, which results in low resistivity, ρ, sinceρ=1/qμN.

The ability to obtain regions that can support high voltage with lowresistance compared to similar device structures in other materialsallows embodiments of the present invention to provide resistanceproperties and voltage capability of conventional devices, while usingsignificantly less area for the GaN device. Capacitance, C, scales witharea, approximated as C=εA/t, so the smaller device will have lessterminal-to-terminal capacitance. Lower capacitance leads to fasterswitching and less switching power loss.

These and other embodiments of the invention along with many of itsadvantages and features are described in more detail in conjunction withthe text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional diagram of a GaN-based Schottkybarrier diode (SBD);

FIG. 2 is a graph providing the general I-V characteristics of theGaN-based SBD shown in FIG. 1;

FIG. 3 is an energy band diagram illustrating the interface between theSchottky contact and the epitaxial layer of the GaN-based SBD of FIG. 1;

FIG. 4 is a simplified cross-sectional diagram of a GaN-based SBDaccording to an embodiment of the present invention;

FIG. 5 is an energy band diagram of the interfaces between the Schottkycontact, the surface layer, and the epitaxial layer of the GaN-based SBDillustrated in FIG. 4;

FIG. 6 is a graph providing the general I-V characteristics of theGaN-based SBD illustrated in FIG. 4 as compared with the GaN-based SBDillustrated in FIG. 1;

FIG. 7A is a graph that plots the critical thickness of an n-type AlGaNsurface layer as a function of aluminum mole fraction according to anembodiment of the present invention;

FIG. 7B is a graph that plots the carrier density of a two-dimensionalelectron gas (2DEG) as a function of surface layer thickness accordingto an embodiment of the present invention;

FIG. 8 is an energy band diagram illustrating the interfaces between theSchottky contact, the surface layer, and the epitaxial layer of theGaN-based SBD according to an embodiment of the present invention,showing different sets of bands for different thicknesses of the surfacelayer;

FIG. 9 is an energy band diagram of a graded surface layer, according toone embodiment of the present invention, as compared with a surfacelayer having uniform composition, according to another embodiment of thepresent invention; and

FIG. 10 is a simplified flowchart illustrating a method for fabricatinga Schottky diode utilizing a surface layer as described herein,according to an embodiment of the present invention.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention relates generally to electronic devices. Morespecifically, the present invention relates to techniques for providinga Schottky barrier diode using III-nitride semiconductor materials andhaving a surface layer that modifies the performance of the Schottkybarrier diode. Merely by way of example, the invention has been appliedto methods and systems for manufacturing Schottky barrier diodes usingone or more gallium-nitride (GaN) based epitaxial layers and an aluminumgallium nitride (AlGaN) surface layer. The methods and techniques can beapplied to create diodes for a variety of applications that can benefitfrom the low leakage current associated with the diode.

The speed and efficiency of the Schottky barrier diode (SBD) render suchmetal-semiconductor devices suitable for many applications in today'sworld of modern electronics. Although the simplicity of the SBD's designcan provide for the device's low junction capacitance and ultra-fastswitching action, it can also present some performance-relateddrawbacks. One notable drawback is the fact that SBDs typically have arelatively high leakage current under reverse bias. Additionally,processing techniques can damage the crystal surface of somesemiconductor-based SBDs, thus compromising the quality of the Schottkycontact. Techniques disclosed herein provide for an improved SBDstructure that can offer greater chemical stability during manufactureas well as a reduced reverse leakage current without sacrificing many ofthe characteristics for which the SBD might be used.

FIG. 1 is a simplified cross-sectional diagram of an embodiment of aGaN-based SBD 100. In the illustrated embodiment, a substrate 130, whichwill be a cathode of the GaN-based SBD 100, is an n-type GaN substrate,but different embodiments can include different materials. In otherembodiments, for example, substrates with p-type doping are utilized.Additionally, although a GaN substrate and GaN epitaxial layers areutilized in the GaN-based SBD 100 illustrated in FIG. 1, otherembodiments are not limited to GaN substrates and GaN epitaxial layers.Other III-V materials, in particular, III-nitride materials, areincluded within the scope of the present invention and can besubstituted not only for the illustrated substrate 130, but also forother GaN-based layers and structures described herein, such asepitaxial layer 120. As examples, binary III-V (e.g., III-nitride)materials, ternary III-V (e.g., III-nitride) materials such as InGaN andAlGaN, quaternary III-nitride materials, such as AlInGaN, doped versionsof these materials, and the like are included within the scope of thepresent invention. Additionally, embodiments can use materials having anopposite conductivity type (e.g., p-type or n-type) to provide deviceswith different functionality.

Although some examples relate to the growth of n-type GaN epitaxiallayer(s) doped with silicon, in other embodiments the techniquesdescribed herein are applicable to the growth of highly or lightly dopedmaterial, p-type material, material doped with dopants in addition to orother than silicon such as Mg, Ca, Be, Ge, Se, S, O, Te, Zn, C, and thelike. The substrates discussed herein can include a single materialsystem or multiple material systems including composite structures ofmultiple layers. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

An ohmic contact 140 is electrically coupled to a first surface 133 ofthe substrate 130. The ohmic contact 140 can be one or more layers ofohmic metal that serve as an electrical contact for the cathode of theGaN-based SBD 100. For example, the ohmic contact 140 can comprise atitanium-aluminum (Ti/Al) ohmic metal. Other metals and/or alloys can beused including, but not limited to, aluminum, nickel, gold, combinationsthereof, or the like. In some embodiments, an outermost metal of theohmic contact 140 can include gold, tantalum, tungsten, palladium,silver, or aluminum, combinations thereof, and the like. The ohmiccontact 140 can be formed using any of a variety of methods such assputtering, evaporation, or the like.

Coupled to a second surface 135 of the substrate 130 opposite the firstsurface 133, is an epitaxial layer 120, which provides a drift region ofn-type GaN material for the GaN-based SBD 100. The epitaxial layer 120will therefore have properties such as thickness and dopingconcentration that are determined by the design of the GaN-based SBD100. In typical embodiments, the thickness of the epitaxial layer 120can be between about 1 μm to about 100 μm and the doping concentrationcan be between about 1×10¹⁴ cm⁻³ to about 1×10¹⁷ cm⁻³. In otherembodiments, the thickness and doping concentration are modified asappropriate to the particular application. Additional descriptionrelated to thicknesses, dopant concentrations, and breakdown voltages ofthe drift layer are provided in U.S. patent application Ser. No.13/198,655, filed on Aug. 4, 2011, the disclosure of which is herebyincorporated by reference in its entirety.

The GaN-based SBD 100 further includes a Schottky contact 110electrically coupled to the epitaxial layer 120. In some embodiments, asurface of the epitaxial layer to which the Schottky contact is coupledcan be treated to place it in a condition suitable to create a Schottkybarrier. The Schottky contact 110 comprises one or more Schottky metalsthat are deposited and patterned to form the Schottky contact 110.Examples of Schottky metals include nickel, palladium, platinum,combinations thereof, or the like. The geometry of the Schottky contact110 will be a function of the device geometry for the GaN-based SBD 100,and can vary depending on desired functionality. One of ordinary skillin the art would recognize many variations, modifications, andalternatives.

FIG. 2 is a graph providing the I-V characteristics of the GaN-based SBD100 shown in FIG. 1. The I-V plot 210 shows current of the GaN-based SBD100 as a function of voltage. As illustrated, when compared with theturn-on voltage, V_(t), the GaN-based SBD 100 is able to sustain highvoltages under reverse bias before reaching the breakdown threshold,V_(br). This is not without some leakage current, however, whichincreases with increased reverse bias. The amount of leakage current atthe breakdown threshold V_(br), for example, is indicated by I_(br).Such leakage current is undesirable in most applications.

FIG. 3 is an energy band diagram illustrating the interface between theSchottky contact 110 and the epitaxial layer 120 of the GaN-based SBD100, which can provide additional insight into the cause of the leakagecurrent. The valence band, conduction band, and Fermi level of theepitaxial layer 120 are labeled E_(v), E_(c), and E_(f), respectively.The energy band diagram illustrates the barrier height, φ_(b),associated with injection of electrons from the Schottky contact 110into the n-type GaN material of the epitaxial layer 120 in forward bias.The barrier height φ_(b) also plays a role in suppressing electronleakage from the epitaxial layer 120 to the Schottky contact 110 inreverse bias. Thus, Schottky contacts having a large barrier height 4are generally less susceptible to reverse bias leakage current. Withthis principle in mind, additional measures can be taken to reduce thereverse bias leakage current of GaN-based SBD 100.

FIG. 4 illustrates an embodiment of a GaN-based SBD 400 according to anembodiment of the present invention. The features of the GaN-based SBD400, such as physical dimensions, doping levels, and the like, aresimilar to corresponding features of the GaN-based SBD 100 of FIG. 1.Additionally, similar to the GaN-based SBD 100 of FIG. 1, theimprovements illustrated by the improved GaN-based SBD 400 illustratedin FIG. 4 also may be implemented in diodes comprising other materials,including other III-nitride materials. Referring to FIG. 4, theGaN-based SBD 400 includes an additional feature not illustrated in FIG.1: a surface layer 410, also referred to as a surface region, disposedbetween the epitaxial layer 120 and the Schottky contact 110.

The surface layer 410 can include a material with a larger band gap thanthe epitaxial layer 120 and the same conductivity type as the epitaxiallayer. This can increase the barrier height of the Schottky barrier,which may reduce the reverse bias leakage current of GaN-based SBD 100.The surface layer 410 can include any of a variety of materials, whichcan vary depending on desired functionality, manufacturing concerns, andother factors. In embodiments where the epitaxial layer 120 comprises aGaN material, for example, the epitaxial layer 120 can include anycombination aluminum gallium nitride (AlGaN), aluminum nitride (AlN),aluminum indium nitride (AlInN), aluminum gallium indium nitride(AlGaInN), and the like. Embodiments having an epitaxial layer includingmaterials other than GaN may utilize different materials in the surfacelayer. To avoid a rectifying junction between the surface layer 410 andthe epitaxial layer, the surface layer 410 can be doped such that it hasthe same electrical conductivity type as the epitaxial layer. Forexample, in one embodiment, the surface layer comprises an n-type AlGaNlayer, and the epitaxial layer 120 comprises an n-type GaN epitaxiallayer. In other embodiments, the surface layer 410 is undoped. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

Among other advantages, the surface layer 410 can be more stable thanthe epitaxial layer 120. In the case where the surface layer 410comprises n-type AlGaN and the epitaxial layer 120 comprises n-type GaN,for example, the n-type AlGaN surface layer offers enhanced chemicalstability over n-type GaN. This provides a more durable semiconductorsurface capable of withstanding the potentially damaging effects oflithographic and/or other processing. Ultimately, this can provide acleaner interface for Schottky barrier, resulting in a better performingSBD.

As alluded to earlier, the inclusion of the surface layer 410 can impactthe band height of the height of the Schottky barrier formed between theSchottky contact 110 and the surface layer 410. FIG. 5 is an energy banddiagram illustrating the interfaces between the Schottky contact 110,the surface layer, and the epitaxial layer 120 of the improved GaN-basedSBD 400. When compared with FIG. 3, FIG. 5 illustrates how the surfacelayer 410 modifies the electrical behavior of the improved GaN-based SBD400. For example a modified barrier height, φ′_(b), of the improvedGaN-based SBD 400 is larger than the corresponding barrier height φ_(b)of the GaN-based SBD 100 of FIG. 1.

FIG. 6 is a graph illustrating the I-V characteristics of the GaN-basedSBD 400 of FIG. 4 as compared with the GaN-based SBD 100 of FIG. 1. TheI-V curves 610 and 210 show the behavior of the current of the GaN-basedSBD 400 and the current of the GaN-based SBD 100, respectively, as afunction of voltage. As illustrated, the inclusion of the epitaxiallayer 120 enables the GaN-based SBD 400 to exhibit an increased reversebias breakdown voltage V′_(br) over the corresponding reverse biasbreakdown voltage V_(br) of the GaN-based SBD 100. As illustrated inFIG. 6, the epitaxial layer 120 included in GaN-based SBD 400 may resultin a higher turn-on voltage V′_(t) in comparison with the turn-onvoltage V_(t) of the GaN-based SBD 100. The amount of increase in thereverse bias breakdown voltage V′_(br) and/or turn-on voltage V′_(t) ofthe improved GaN-based SBD 400 over corresponding voltages of theGaN-based SBD 100 can vary, depending on the physical properties (e.g.,material types, concentrations, etc.) of the materials included in thesurface layer 410, among other factors.

Due to Fermi-level pinning and polarization differences, in addition todifferences in the respective band gaps of the surface layer 410 and theepitaxial layer 120, the performance of the improved GaN-based SBD 400can be impacted by the thickness 420 (FIGS. 4 and 5) of the surfacelayer 410 where the epitaxial layer 120 includes a polar material. Atwo-dimensional electron gas (2DEG), for example, is known to accumulateat certain heterojunctions, such as the heterojunction between thesurface layer 410 and the epitaxial layer 120 in certain embodiments ofthe present invention. Such a 2DEG can impact the operation of theGaN-based SBD 400.

FIG. 7A shows a curve 710 of critical thickness in nanometers (nm) as afunction of aluminum (Al) mole fraction, and FIG. 7B shows a curve 720illustrating the relation between the critical thickness (indicated hereas t_(critical)) and the 2DEG electron density, n_(2DEG). Calculationsof n_(2DEG) as a function of critical thickness in heterojunctionscomprising AlGaN and GaN (or other suitable materials) can be made usingknown methods. One such method is provided with equation 16a in 0.Ambacher et al. “Two dimensional electron gases induced by spontaneousand piezoelectric polarization in undoped and doped AlGaN/GaNheterostructures,” Journal of Applied Physics 87, 334 (2000), which ishereby incorporated by reference. The plots in FIGS. 7A and 7B ofcritical thickness as a function of Al mole fraction and the electrondensity of the 2DEG as a function of layer thickness are computed for anepitaxial layer 120 including polar (e.g., c-plane) n-type GaN materialand an AlGaN surface layer 410, but other plots can be computed forother materials as will be evident to one of skill in the art.

FIG. 8 is an energy band diagram of the interfaces between the Schottkycontact 110, the surface layer 410, and the epitaxial layer 120 of theGaN-based SBD 400. A first set of bands 810 indicates where the surfacelayer 410 has a first thickness 420-1 that is less than t_(critical),and a second set of bands 830 indicates where the surface layer 410 hasa second thickness 420-2 that is greater than t_(critical). Asillustrated, when the surface layer thickness exceeds the value denotedt_(critical), the Fermi level pinning on the surface forces the n-typeGaN's energy bands downward, confining electrons to a substantiallytriangular potential well 850 at the n-type GaN/n-type AlGaN interfacein which the charge is determined by polarization effects. The 2DEG thatdevelops at the n-type GaN/n-type AlGaN interface also may be affectedby processes that passivate the surface. The 2DEG is undesirable in mostembodiments. Thus, precautions can be made such that creation of the2DEG is avoided.

Referring again to FIG. 7A, the critical thickness (i.e., t_(critical))of the n-type AlGaN surface layer 410 is shown to bereversely-proportional to the Al mole fraction of the n-type AlGaNsurface layer 410. In other words, the more aluminum included in then-type AlGaN surface layer 410, the thinner the n-type AlGaN surfacelayer 410 should be to prevent a 2DEG from forming. Embodiments can varywidely, depending the desired stoichiometry of the n-type AlGaN surfacelayer 410, among other factors. In some embodiments, for example, then-type AlGaN surface layer 410 has a thickness of between 0.5 nm and 15nm. As particular example, the n-type AlGaN surface layer 410 thicknesscan range from about 1 nm and about 3 nm. Other embodiments can includean n-type AlGaN surface layer 410 with a thickness greater than 15 nm orless than 0.5 nm. According to embodiments of the present invention, then-type AlGaN surface layer 410 described herein prevents a 2DEG fromforming at the interface between the AlGaN surface layer 410 and then-type GaN epitaxial layer 120. In some embodiments, the AlGaN surfacelayer 410 can be a few monolayers in thickness.

In alternative embodiments, the composition of the n-type AlGaN surfacelayer 410 can be graded as a function of thickness. FIG. 9 is an energyband diagram of such a graded surface layer 410 as compared with asurface layer 410 having uniform composition. A first set of bands 910correspond to a surface layer 410 having uniform composition, and asecond set of bands 920 correspond to a graded surface layer 410.Current technology allows the profile of an n-type AlGaN surface layer410 to be continuously graded so as to eliminate an abrupt compositionstep, thereby avoiding the formation of a 2DEG. For example, theconcentration of aluminum in the n-type AlGaN surface layer 410 canincrease as the distance to the Schottky contact 110 decreases. Thisresults in the second set of bands 920 having a smoother profile, whichis less susceptible to the formation of a 2DEG.

It will be understood that the materials with which an epitaxial layeris composed can have a great impact on any requirements concerning thethickness of a surface layer. For instance, nonpolar materials (e.g.,a-plane and/or m-plane GaN) can be utilized in the epitaxial layer. Insuch a case, because there are no polarization effects that could resultin the formation of a 2DEG, there are no associated maximum thicknessrequirements for the corresponding surface layer. Thus, rather thanlimiting the surface layer to as little as a few monolayers or less, aSchottky diode having an epitaxial region comprising nonpolar materialscan have a surface layer of a far larger thickness, limited only byfactors that do not involve the formation of a 2DEG (e.g., latticemismatch that could result in a risk of physical cracking of a materiallayer). One of ordinary skill in the art will recognize that materialsand/or conductivity types other than those discussed above may beutilized.

FIG. 10 is a simplified flowchart illustrating a method 1000 forfabricating a Schottky diode using III-nitride materials according to anembodiment of the present invention. The method 1000 includes providinga III-nitride substrate (1010). Referring to FIG. 4, for example, a bulkn-type GaN substrate 130 is provided. The III-nitride substrate can becharacterized by a first surface and a second surface, where the secondsurface opposes the first surface. The properties of the III-nitridesubstrate, such as thickness, dopant concentration, and the like, canvary depending on desired functionality, among other concerns.

The method 1000 further includes forming an ohmic contact electricallycoupled to the first surface of the III-nitride substrate (1020). Insome embodiments, a single metallic layer is used to form the ohmiccontact. Thicknesses or materials can vary to provide the ohmicproperties appropriate to the device features. In other embodiments,multiple metallic layers (i.e., a multi-layer metallic structure) and/ormultiple metals are utilized to form the ohmic contact. It will beunderstood that, in other embodiments, the formation of the epitaxiallayers and ohmic contact can occur at different points in thefabrication of a Schottky diode, depending on manufacturingconsiderations.

An III-nitride epitaxial region coupled to the second surface of theIII-nitride substrate is formed (1030). The epitaxial region, which canbe a component of a drift region for the Schottky diode, can be formedby epitaxial growth on the III-nitride substrate. Furthermore, theepitaxial region can have any of a variety of physical properties, suchas thickness, dopant concentration, polarity, and the like, depending onthe desired functionality of the Schottky diode. As discussedpreviously, the thickness of the epitaxial region can be relativelylarge to accommodate high-power applications.

The method 1000 further includes forming a surface region coupled to theIII-nitride epitaxial region (1040). As shown in FIG. 4, the surfaceregion can be coupled to a surface of the III-nitride epitaxial regionthat is substantially opposite from the surface of the III-nitrideepitaxial region coupled to the III-nitride substrate. Additionally, thesurface region can be formed any of a variety of ways. For example, amaterial can be introduced into the III-nitride epitaxial region to formthe surface region (e.g., introducing Al into a GaN epitaxial region toform an AlGaN surface region). Additionally or alternatively, thesurface region can comprise a layer of material formed (e.g., grown,deposited, etc.) on the III-epitaxial region. As indicated previously,embodiments can include a surface region comprising a material includingaluminum (e.g., AlGaN, AN, AlInN, and the like), and the concentrationof aluminum can be such that the aluminum mole fraction is highest nearthe surface of the surface region on which the Schottky contact issubsequently formed. Stated another way, the aluminum mole fractionincreases as distance to the Schottky contact decreases.

The method 1000 also includes forming the Schottky contact electricallycoupled to the surface region (1050). As indicated previously, theSchottky contact can comprise one or more Schottky metals that aredeposited and patterned to form the Schottky contact on the surfaceregion. Such metals include nickel, palladium, platinum, combinationsthereof, or the like. In some embodiments, a surface of the surfaceregion to which the Schottky contact is coupled can be treated to placeit in a condition suitable to create the Schottky barrier. However,because the surface region may have increased chemical stability ascompared with the III-nitride epitaxial region, there may be no need forsuch treatment.

It should be appreciated that the specific steps illustrated in FIG. 10provide a particular method of fabricating a Schottky diode according toan embodiment of the present invention. Other sequences of steps mayalso be performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 10 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

One of ordinary skill in the art would recognize many variations,modifications, and alternatives to the examples provided herein. Asillustrated herein, physical characteristics and properties of theSchottky diodes described herein can vary, depending on desiredfunctionality of the Schottky diode. For instance, the material(s) usedin the surface layer can vary, providing different band gaps that canultimately impact the performance of the Schottky diode by altering thereverse bias breakdown voltage, leakage current, and/or the turn-onvoltage. Additionally or alternatively, conductivity types of theexamples provided herein can be reversed (e.g., replacing an n-typesemiconductor material with a p-type material, and vice versa),depending on desired functionality. Moreover, embodiments providedherein using GaN can use other III-nitride materials in addition or asan alternative to GaN. Other variations, alterations, modifications, andsubstitutions are contemplated.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A method of fabricating a Schottky diode usinggallium nitride (GaN) materials, the method comprising: providing ann-type GaN substrate having a first surface and a second surface, thesecond surface opposing the first surface; forming an ohmic metalcontact electrically coupled to the first surface of the n-type GaNsubstrate; forming an n-type GaN epitaxial layer coupled to the secondsurface of the n-type GaN substrate; forming an n-type aluminum galliumnitride (AlGaN) surface layer coupled to the n-type GaN epitaxial layer,wherein the AlGaN surface layer has a thickness which is less than acritical thickness, wherein the critical thickness is determined basedon an aluminum mole fraction of the AlGaN surface layer; and forming aSchottky contact electrically coupled to the n-type AlGaN surface layer,wherein, during operation, an interface between the n-type GaN epitaxiallayer and the n-type AlGaN surface layer is substantially free from atwo-dimensional electron gas.
 2. The method of claim 1, wherein then-type GaN epitaxial layer comprises a nonpolar GaN material.
 3. Themethod of claim 1, wherein the n-type GaN epitaxial layer comprises apolar GaN material.
 4. The method of claim 3, wherein the thickness ofthe n-type AlGaN surface layer is between 0.5 nm and 15 nm.
 5. Themethod of claim 4, wherein the thickness of the n-type AlGaN surfacelayer is between 1 nm and 3 nm.
 6. The method of claim 3, wherein thegraded aluminum mole fraction of the n-type AlGaN surface layer isgraded as a function of thickness.
 7. The method of claim 6, wherein thealuminum mole fraction increases as a distance to the second metallicstructure decreases.
 8. The method of claim 1, wherein the n-type GaNepitaxial layer has a larger band gap than the n-type GaN epitaxiallayer.
 9. A semiconductor device comprising: an n-type GaN substratehaving a first surface and a second surface, the second surface opposingthe first surface; an ohmic metal contact electrically coupled to thefirst surface of the n-type GaN substrate; an n-type GaN epitaxial layercoupled to the second surface of the n-type GaN substrate; an n-typealuminum gallium nitride (AlGaN) surface layer coupled to the n-type GaNepitaxial layer, wherein the AlGaN surface layer has a thickness whichis less than a critical thickness, wherein the critical thickness isdetermined based on an aluminum mole fraction of the AlGaN surfacelayer; and a Schottky contact electrically coupled to the n-type AlGaNsurface layer, wherein, during operation, an interface between then-type GaN epitaxial layer and the n-type AlGaN surface layer issubstantially free from a two-dimensional electron gas.
 10. The deviceof claim 9, wherein the n-type GaN epitaxial layer comprises a nonpolarGaN material.
 11. The device of claim 9, wherein the n-type GaNepitaxial layer comprises a polar GaN material.
 12. The device of claim11, wherein the thickness of the n-type AlGaN surface layer is between0.5 nm and 15 nm.
 13. The device of claim 12, wherein the thickness ofthe n-type AlGaN surface layer is between 1 nm and 3 nm.
 14. The deviceof claim 11, wherein the graded aluminum mole fraction of the n-typeAlGaN surface layer is graded as a function of thickness.
 15. The deviceof claim 14, wherein the aluminum mole fraction increases as a distanceto the second metallic structure decreases.
 16. The device of claim 9,wherein the n-type GaN epitaxial layer has a larger band gap than then-type GaN epitaxial layer.